Industrial usage of metal parts manufactured by the compaction and sintering of metal powder compositions is expanding rapidly into a multitude of areas. Manufacture of parts with metal powder compositions provides substantial benefits in comparison to use of a molten alloy in the manufacturing process. In the manufacture of such parts, iron or steel particulate powders are often admixed with at least one other alloying element that is also in particulate form. These alloying elements permit the attainment of higher strength and other mechanical properties in the final sintered part. The alloying elements typically differ from the base iron or steel powders in particle size, shape and density. For example, the average particle size of the iron-based powders is typically about 70-100 microns, or more, while the average particle size of most alloying ingredients can be less than about 20 microns, more often less than about 15 microns, and in some cases less than about 5 microns. The alloying powders are purposely used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solid-state diffusion during the sintering operation.
The disparity in particle size can lead to problems such as segregation and dusting of the finer alloying particles during transportation, storage, and use. Although the iron and alloy element powders are initially admixed into a homogeneous powder, the dynamics of handling the powder mixture during storage and transfer can cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix. The normal forces of gravity, particularly where the alloying powder is denser than the iron powder, cause the alloying powder to migrate downwardly toward the bottom of the mixture's container, resulting in a loss of homogeneity of the mixture, or segregation. On the other hand, air currents which can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can, in the phenomenon known as dusting, escape the mixture entirely, resulting in a decrease in the concentration of the alloy element.
Various organic binders have been used to bind or "glue" the finer alloying powder to the coarser iron-based particles to prevent segregation and dusting for powders to be compacted at ambient temperatures. For example, U.S. Pat. No. 4,483,905 to Engstrom teaches the use of a binding agent that is broadly described as being of "a sticky or fat character" in an amount up to about 1% by weight of the powder composition. U.S. Pat. No. 4,676,831 to Engstrom discloses the use of certain tall oils as binding agents. Also, U.S. Pat. No. 4,834,800 to Semel discloses the use of certain film-forming polymeric resins that are generally insoluble in water as binding agents. These binders are effective in preventing segregation and dusting, but like any of the other organic binders used by the prior art, they can adversely affect the compressibility of the powder even when present in only small amounts.
The "compressibility" of a powder blend is a measure of its performance under various conditions of compaction. In the art of powder metallurgy, a powder composition is generally compacted under great pressure in a die, and the compacted "green" part is then removed from the die and sintered. It is recognized in this art that the density, and usually the strength, of this green part vary directly with the compaction pressure. In terms of "compressibility", one powder composition is said to be more compressible than another if, at a given compaction pressure, it can be pressed to a higher green density, or alternatively, if it requires less compaction pressure to attain a specified green density.
It is also known now that there are advantages to compressing powder compositions at elevated temperatures. See, for example, U.S. Pat. No. 5,154,881 to Rutz et al., which discloses enhancement in post-compaction properties such as green density and green strength due to the warm compaction procedure. The compaction at elevated temperatures requires the presence of a lubricant to facilitate ejection of the compacted part from the die. Although the green density of a compacted part generally increases with the compaction pressure, so do the friction forces that must be overcome to remove the compacted part from the die. The presence of the lubricant keeps the friction force from exceeding a level at which significant die wear would occur. Not all lubricants conventionally used in powder metallurgical processes retain their properties if compaction is performed at elevated temperatures. Rutz et al. disclose an amide lubricant that is suitable for warm compaction procedures.